(1) Field of the Invention
The present invention relates to a color image sensor for use in an optical scanning system for hard copy images, in particular it relates to a color image sensor fabricated using optical waveguides and a production method of an optical waveguide array for use therein.
(2) Description of the Background Art
In recent years, with the increased demand for image reading in facsimile machines, image scanners, digital copiers etc., there has been demand for high-performance and compact image sensors which convert image information into electric signals. Conventionally, image sensors which sequentially scan linear sections of the image include a contact type sensor (also called an isometric type sensor) which picks up an isometrically focused image using an optical system having a sensor length corresponding to the width of the original, and a reduction type sensor which picks up a focused image using an image reducing optical system with a sensor length shorter than the width of the original. The contact type image sensor has a structure in which reflected light from the original is guided to a photoelectric converting element through a SELFOC lens array. The reduction optical type image sensor has a structure in which reflected light from the original is guided to a photoelectric converting element using a single lens and mirrors (to be referred to hereinbelow as lens reduction optical type image sensor).
In the field of image sensors, evolution into color is being promoted. Techniques for color recognition include: a method of switching between three types of color filters; a method of switching between three colors of light sources; a method of using a sensor with color separation filters; and the like. In the technique using different color filters, a single line CCD is used for scanning and the light is turned on and off for each color at a certain point on the optical path. In the technique using different light sources, three color light sources are activated in turn for each line. A sensor with color separation filters uses RGB filters attached in the front of the photoelectric converting element; for example, a three-line color CCD in which three color filters are applied to respective lines.
Applied to the lens reduction optical image sensors are the method of using three types of color filters, the method of using a sensor with color separation filters, the method of switching between three colors of light sources, and the like. The contact type image sensor can use the light source changing method, the method of using a sensor with color separation filters, and the like.
Since the SELFOC lens array is made so as to be in close contact with the photoelectric converting element in a contact type image sensor, the sensor of this type has the advantage of being compact in the light propagating direction. However, this sensor needs to have the same sensor length as the width of the original, thus making the price of the photoelectric converting element considerably expensive and increasing the price of the image sensor. Further, the sensor of this type has a shallow depth of field, presenting drawbacks in that any variation in the distance between the original and the SELFOC lens array is liable to cause a blurred out-of-focus image.
A reduction optical type image sensor has a good depth of field and reduces the images by use of a lens. This feature makes the photoelectric converting element compact, presenting an advantage in the cost of the whole image sensor. However, since the reduction optics use a focusing process and mirror reflection, needing a greater optical distance, there is a limit to its compactness with respect to the light propagating direction. Further, the adjustment of the optical system is complicated, resulting in a lack of mechanical stability and reliability.
To deal with this, an optical reduction type image sensor which uses optical waveguides has been developed. This optical reduction type image sensor of optical waveguide type performs reduction of images using the reflected light from the original surface and comprises an array of microlenses having a width the same as the original surface, an optical waveguide substrate formed with a plurality of waveguides conducting light focused by the lens, and a photoelectric converting element array which converts the light introduced from the multiple optical waveguides into electric signals. This configuration makes the device cheaper, more compact and thinner, needing no complex adjustment of the optical system (Japanese Patent Application Laid-Open Hei 7 No. 301730).
The color reading method for this optical waveguide reduction optical type image sensor can be realized by shaping a single waveguide into three branches with RGB (red, green and blue) filters attached to their exits. In this case, two types of arrangements have been proposed: one in which three branches are formed within the waveguide substrate; and the other in which three branches are formed perpendicular to the substrate (Japanese Patent Application Laid-Open Hei 3 No. 171,865).
However, the above optical waveguide type optical reduction type image sensor of the three branched type involves the following problems in the production. First, although it is easy to create two branches for dividing optical energy into halves, it is difficult to divide the waveguide into three branches because the pitch becomes shorter. Even if three branches can be formed successfully within the substrate plane, all the output ends must be provided with RGB color filters in proper order; this makes for a complicated assembly process. When three branches are arranged perpendicular to the substrate, a linear RGB color filter can be used making the assembly simpler, but no appropriate mass production method for producing three branches in the perpendicular direction has been found at present.